Add EventPayload::extractValue for NativeAnimated (#49988)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49988

NativeAnimated currently depends on folly::dynamic event payloads for event-driven animations. While some events (e.g., ScrollEvent.h) have an `asDynamic` implementation, not all events do. In practice, NativeAnimated just needs to be able to extract an numeric value from a path to drive an animation.

Rather than converting events to dynamic, or otherwise special casing event handling, this change allows arbitrary payloads to implement `EventPayload::extractValue` to retrieve JS property path values directly from events, without intermediate conversions to dynamic.

## Changelog

[Internal]

Reviewed By: javache

Differential Revision: D71046682

fbshipit-source-id: 3544335ff9d50da87ced015de587b97204173b57

packages/react-native/ReactCommon/react/renderer/components/scrollview/ScrollEvent.cpp

@@ -72,6 +72,21 @@ folly::dynamic ScrollEvent::asDynamic() const {
   return metrics;
 };
 
+std::optional<double> ScrollEvent::extractValue(
+    const std::vector<std::string>& path) const {
+  if (path.size() == 1 && path[0] == "zoomScale") {
+    return zoomScale;
+  } else if (path.size() == 2 && path[0] == "contentOffset") {
+    if (path[1] == "x") {
+      return contentOffset.x;
+    } else if (path[1] == "y") {
+      return contentOffset.y;
+    }
+  }
+
+  return EventPayload::extractValue(path);
+}
+
 EventPayloadType ScrollEvent::getType() const {
   return EventPayloadType::ScrollEvent;
 }

packages/react-native/ReactCommon/react/renderer/components/scrollview/ScrollEvent.h

@@ -36,6 +36,9 @@ struct ScrollEvent : public EventPayload {
    */
   jsi::Value asJSIValue(jsi::Runtime& runtime) const override;
   EventPayloadType getType() const override;
+
+  std::optional<double> extractValue(
+      const std::vector<std::string>& path) const override;
 };
 
 struct ScrollEndDragEvent : public ScrollEvent {

packages/react-native/ReactCommon/react/renderer/core/EventPayload.h

@@ -10,6 +10,8 @@
 #include <jsi/jsi.h>
 
 #include <react/renderer/core/EventPayloadType.h>
+#include <optional>
+#include <vector>
 
 namespace facebook::react {
 
@@ -33,6 +35,17 @@ struct EventPayload {
    * in `EventPayloadType` and return it from its overriden `getType()` method.
    */
   virtual EventPayloadType getType() const = 0;
+
+  /**
+   * Used to extract numeric values from the event payload based on
+   * property path names as they will exist in JavaScript. This can
+   * be used in conjunction with listeners on EventEmitters to do
+   * things like drive native animations.
+   */
+  virtual std::optional<double> extractValue(
+      const std::vector<std::string>& /* path */) const {
+    return std::nullopt;
+  }
 };
 
 using SharedEventPayload = std::shared_ptr<const EventPayload>;